RND(9) BSD Kernel Developer's Manual RND(9)NAME
RND, rnd_attach_source, rnd_detach_source, rnd_add_data, rnd_add_uint32 —
functions to make a device available for entropy collection
SYNOPSIS
#include <sys/rnd.h>
void
rnd_attach_source(rndsource_element_t *rnd_source, char *devname,
uint32_t source_type, uint32_t flags);
void
rnd_detach_source(rndsource_element_t *rnd_source);
void
rnd_add_data(rndsource_element_t *rnd_source, void *data, uint32_t len,
uint32_t entropy);
void
rnd_add_uint32(rndsource_element_t *rnd_source, uint32_t datum);
DESCRIPTION
These RND functions make a device available for entropy collection for
the kernel entropy pool, which provides key material for the cprng(9) and
rnd(4) (/dev/random) interfaces.
Ideally the first argument rnd_source of these functions gets included in
the devices' entity struct, but any means to permanently (statically)
attach one such argument to one incarnation of the device is ok. Do not
share rnd_source structures between two devices.
rnd_attach_source(rndsource_element_t *rnd_source, char *devname,
uint32_t source_type, uint32_t flags)
This function announces the availability of a device for
entropy collection. It must be called before the source struct
pointed to by rnd_source is used in any of the following func‐
tions.
devname is the name of the device. It is used to print a mes‐
sage (if the kernel is compiled with ``options RND_VERBOSE'')
and also for status information printed with rndctl(8).
source_type is RND_TYPE_NET for network devices, RND_TYPE_DISK
for physical disks, RND_TYPE_TAPE for a tape drive,
RND_TYPE_TTY for a tty, and RND_TYPE_RNG for a random number
generator. RND_TYPE_UNKNOWN is not to be used as a type. It
is used internally to the rnd system.
flags are the logical OR of RND_FLAG_NO_COLLECT (don't collect
or estimate) RND_FLAG_NO_ESTIMATE (don't estimate) to control
the default setting for collection and estimation. Note that
devices of type RND_TYPE_NET default to RND_FLAG_NO_ESTIMATE.
rnd_detach_source(rndsource_element_t *rnd_source)
This function disconnects the device from entropy collection.
rnd_add_uint32(rndsource_element_t *rnd_source, uint32_t datum)
This function adds the value of datum to the entropy pool. No
entropy is assumed to be collected from this value, it merely
helps stir the entropy pool. All entropy is gathered from jit‐
ter between the timing of events.
Note that using a constant for datum does not weaken security,
but it does not help. Try to use something that can change,
such as an interrupt status register which might have a bit set
for receive ready or transmit ready, or other device status
information.
To allow the system to gather the timing information accu‐
rately, this call should be placed within the actual hardware
interrupt service routine. Care must be taken to ensure that
the interrupt was actually serviced by the interrupt handler,
since on some systems interrupts can be shared.
This function loses nearly all usefulness if it is called from
a scheduled software interrupt. If that is the only way to add
the device as an entropy source, don't.
If it is desired to mix in the datum and to add in a timestamp,
but not to actually estimate entropy from a source of random‐
ness, passing NULL for rnd_source is permitted, and the device
does not need to be attached.
rnd_add_data(rndsource_element_t *rnd_source, void *data, uint32_t len,
uint32_t entropy)
adds (hopefully) random data to the entropy pool. len is the
number of bytes in data and entropy is an "entropy quality"
measurement. If every bit of data is known to be random,
entropy is the number of bits in data.
Timing information is also used to add entropy into the system,
using inter-event timings.
If it is desired to mix in the data and to add in a timestamp,
but not to actually estimate entropy from a source of random‐
ness, passing NULL for rnd_source is permitted, and the device
does not need to be attached.
INTERNAL ENTROPY POOL MANAGEMENT
When a hardware event occurs (such as completion of a hard drive transfer
or an interrupt from a network device) a timestamp is generated. This
timestamp is compared to the previous timestamp recorded for the device,
and the first, second, and third order differentials are calculated.
If any of these differentials is zero, no entropy is assumed to have been
gathered. If all are non-zero, one bit is assumed. Next, data is mixed
into the entropy pool using an LFSR (linear feedback shift register).
To extract data from the entropy pool, a cryptographically strong hash
function is used. The output of this hash is mixed back into the pool
using the LFSR, and then folded in half before being returned to the
caller.
Mixing the actual hash into the pool causes the next extraction to return
a different value, even if no timing events were added to the pool.
Folding the data in half prevents the caller to derive the actual hash of
the pool, preventing some attacks.
In the NetBSD kernel, values should be extracted from the entropy pool
only via the cprng(9) interface. Direct access to the entropy pool is
unsupported and may be dangerous. There is no supported API for direct
access to the output of the entropy pool.
FILES
These functions are declared in src/sys/sys/rnd.h and defined in
src/sys/dev/rnd.c.
SEE ALSOrnd(4), rndctl(8), cprng(9)HISTORY
The random device was introduced in NetBSD 1.3.
AUTHORS
This implementation was written by Michael Graff <explorer@flame.org>
using ideas and algorithms gathered from many sources, including the
driver written by Ted Ts'o.
BUGS
The only good sources of randomness are quantum mechanical, and most com‐
puters avidly avoid having true sources of randomness included. Don't
expect to surpass "pretty good".
BSD December 17, 2011 BSD
